1. Field of the Invention
The present invention relates to removal of coatings from surfaces, and more specifically, it relates to the use of a high power, short pulse laser system in a compact unit for surface coating removal.
2. Description of Related Art
Pulsed high intensity light can be used to remove paint and other types of layered coatings from surfaces such as masonry, wood, concrete, metal signs, metal beams and most other materials used in the construction of buildings, bridges, statues, roads, boats, airplanes, etc. The source of the radiation may be coherent or incoherent such as from flashlamps, arc-lamps or lasers. If the light pulse is controlled properly it can remove most coatings without damaging underlying surfaces. Several mechanisms for coating removal operate depending on the absorption mechanisms and the pulse duration. Flashlamp or laser-based removal systems with pulse durations in the range from 1 .mu.s to 300 .mu.s remove coatings through a thermal process (vaporization) and have been used in the field of art conservation for many years. Patents employing the flashlamp method for industrial paint removal have since been developed (U.S. Pat. No. 5,281,798). The combination of flashlamp heating with an abrasive particle flux has also been used for aircraft paint stripping (U.S. Pat. No. 5,194,723).
Coating removal without damaging the substrate requires pulse durations substantially shorter than a millisecond, such that the deposition of energy into the coating is terminated before significant heat is conducted into the substrate. This process has been previously described by Woodroffe (U.S. Pat. No. 4,756,765) and also by Lovoi (U.S. Pat. No. 4,737,628). In general, lasers are useful in this application because they can produce high energy density short pulses that can be controlled and which may be of nearly arbitrary duration. In the particular case of pulse durations shorter than about 30 to 50 ns, the deposition of energy occurs in a time scale shorter than the acoustic relaxation time in the surface layer. Such pulse durations can usually be produced only by Q-switched lasers. In this situation, the laser-deposited energy initiates a pattern of photoacoustic stress waves which result in removal of the coating through spallation and not through thermal vaporization. The coating material is separated from the surface in the form of a powder or flakes which can be easily removed as with a vacuum system without contaminating the environment. The short pulse mode of removal has been patented by Boquillon et al. (U.S. Pat. No. 5,151,134) in the field of cleaning pollutants from surfaces. For most purposes, the short pulse removal method (pulse duration &lt;30 ns) is the most efficient means for coating removal. The empirical dependence of removal rate on pulse duration has been investigated by Liu and Garmire (Appl. Optics 34, 4409 (1995)).
The systems and methods described in prior art do not address several key issues that limit this technique to specialized areas or small scale applications. Firstly, all methods and devices are based on conventional technologies for producing the radiant energy source (e.g. commercially available Nd:glass or Nd:YAG systems, pulsed TEA CO.sub.2 laser systems, or pulsed flashlamps). These systems are limited to coating removal rates less than approximately 200 ft.sup.2 /hr. In the case of flashlamp and TEA CO.sub.2 lasers, the ablation mode is thermal vaporization, which requires approximately 16 J/cm.sup.2 -mil of laser fluence to remove paint. Q-switched Nd:YAG and Nd:glass pulses are more efficient (around 1 J/cm.sup.2 -mil). For removal rates substantially larger than 200 ft.sup.2 /hr, a coating removal system requires several .times.100 W to several times 1 kW of Q-switched Nd laser power, and 1 kW to several times 10 kW of pulsed TEA CO.sub.2 power.
Flashlamps are inherently more efficient than laser systems and may also be scaled to high average powers, although the radiant energy they produce is far less amenable to precise pulse control, and long distance propagation. The lamp envelope must be placed in close proximity to the surface being cleaned. Commercial Nd:YAG and Nd:glass laser systems are based on master oscillator-power amplifier configurations in which most of the pulsed energy is extracted from a chain of increasingly sized rods or slabs of the gain medium. These designs cannot be scaled to high average power (e.g. by adding more flashlamps and laser rods) without greatly increasing the bulk and/or complexity of the system.
Also associated with the use of a high average power laser system is the need to automate the delivery of the laser energy so that it is used efficiently, effectively and safely. Lasers that deliver high average power optical beams cannot be controlled or directed manually, such as with simple handpieces. Feedback control of the laser power is essential in order to limit the application of laser energy precisely when the surface coating has been removed and the desired underlying layers have been exposed. Feedback sensors for radiant coating removal systems have been described using various means to assess the degree of removal of the surface coating. These include spectral and spatial reflectivity sensors (U.S. Pat. Nos. 4,588,885 and 4,737,628), photoacoustic pressure sensors (U.S. Pat. No. 5,194,723), reflected color intensity sensors (U.S. Pat. No. 5,281,798) and spectral emission sensors (U.S. Pat. No. 5,204,517).
In summary there is a need for a compact laser system with a scalable design that can reach kW power levels, operate at wavelengths from the UV to the near IR, with pulse durations in the range of 10 to 30 ns, and which can be integrated into a system for surface coating removal.